Adolescence is a transitional period between childhood and adulthood, during which most drug use begins. Drug use initiated during adolescence, and in females compared to males, is associated with poorer long-term outcomes, such as increased cognitive deficits and likelihood to transition to dependence. To address our gap in knowledge of the mechanisms that may confer enhanced addiction vulnerability in adolescents and females, it is important to understand the patterns of drug seeking typical of vulnerable populations (e.g. adolescents and females) and the long-lasting drug-induced neuroadaptations and changes in executive functioning. The long-term goal of the studies proposed here is to identify behavioral and neurobiological mechanisms that may underlie heightened vulnerability to drug dependence in adolescent-onset and female methamphetamine (METH) users. The experiments will employ behavioral, pharmacological, and immunohistochemical techniques in a rat model to assess two specific aims. Aim 1 will characterize age- and sex-specific patterns of maladaptive drug seeking and investigate a mechanism of heightened relapse by examining aspects of intravenous METH self-administration, such as responding when METH is unavailable, motivation to procure METH, extinction responding, and the modulation of cued reinstatement by 5-HT2CR manipulation. Aim 2 will determine the impact of METH self-administration on cognitive flexibility and assess a mechanism - altered 5- HT2CRs in the orbitofrontal cortex - through which the observed deficits may manifest. Fulfillment of the training goals, which include acquisition of a diverse skill set i foundational neurobiological techniques and professional development in the field of addiction neuroscience, will be attained through a combination of coursework and readings, hands-on training with individuals who have expertise in the proposed techniques, attendance at workshops, seminars, and national conferences, and monthly meetings with the training team. The results of the proposed research will move the field toward a clearer understanding of the antecedents and consequences of drug use that confer enhanced addiction vulnerability in at-risk populations. The results of the proposed experiments could be used to facilitate development of individualized prevention and treatment strategies for females and adolescent users, who are at heightened risk to transition to dependence.